When you minimize a fullscreen window with #showMinimized() and then re-open the window from your task-bar, the window is not restored into fullscreen mode.

Following QWindowStateChangeEvent are issued on #showMinimized():

1. WindowMinimized|WindowFullScreen
2. WindowNoState
3. WindowMinimized

It looks like this issue not reproducible on my environment only since it has been reported for KDE KRDC by multiple users: https://bugs.kde.org/show_bug.cgi?id=361570

Note: the following example works fine with Qt 4.8.7, but fails with Qt 5.5.1.

#include <QApplication>
#include <QPushButton>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QPushButton *button = new QPushButton("Minimize");
    QObject::connect(button, SIGNAL(pressed()), button, SLOT(showMinimized()));
    button->showFullScreen();
    return app.exec();
}